A valve timing controller is provided in a power transmission path between a driving shaft and a driven shaft of an internal combustion engine to adjust a valve timing of an intake valve or an exhaust valve. The valve timing controller includes a driving unit which outputs rotational power, a first rotating body which rotates synchronously with the driving shaft, a second rotating body which is provided to an end of the driven shaft, and a relative rotating portion which is provided in the first rotating body. The relative rotating portion rotates relative to the first rotating body, whereby a relative rotation phase between the first rotating body and the second rotating body is varied. The relative rotating portion shown in JP 04200377 B2 is configured by a planetary gear. The relative rotating portion shown in DE 102014211518 A1 is configured by a strain wave gear.
The relative rotating portion is movable relative to the first rotating body in an axial direction. In order to avoid a disengagement of the relative rotating portion, a cover fixed to the first rotating body supports the relative rotating portion.
When a valve timing is adjusted, the relative rotating portion rotates relative to the cover. Thus, a sliding movement between the relative rotating portion and the cover may deteriorate a power transmission efficiency and may cause an abrasive wear.